Field
Various signal processing techniques may benefit from appropriate handling. For example, certain signal processors may benefit from quarter wavelength unit delay and complex weight coefficient continuous-time filters.
Description of the Related Art
Various modulation methods are used to transmit wanted information through communication media to the receiver. Continuous-mode signals can be used within the signal transmission whether the transmission is binary or amplitude and/or phase modulated signal. The continuous-time domain signal can be distorted within the transmission. Sometimes the transceiver analog domain parts cause most of the signal linear and nonlinear distortion.
The signal distortion may need to be minimized through optimizing circuit and system level design. Further minimization can be done through adaptive equalization, possibly using both digital signal processing (DSP) and analog signal processing (ASP). ASP can be useful within full-duplex radios where the strong transmission (TX) signal is transmitted simultaneously at the same frequency as the possibly weaker reception (RX) signal is received.
In practice, part of the TX signal leaks to the receiver signal paths and sums up with the wanted received RX signal. Removing the leaked TX signal from the receiver paths can aid in correct reception of the wanted RX signal, namely reception with sufficient signal purity.
Cancellation of the leaked TX signal, however, is not an easy task. One complication may be due to the leakage TX signal being affected by per frequency amplitude, phase and delay changes during the leakage mechanisms. The leakage mechanisms can include conductions, reflections, radiation, and so on.
Furthermore, the mentioned frequency response characteristics may significantly differ over broadband. Therefore, the ASP-circuit may be useful within full-duplex radio. The ASP unit can be used in feedforward manner a copy/sample of TX signal can be taken. Signal characteristics of the copy can be altered with ASP and the altered TX signal can be fed forward to the receiver path so that the unwanted leakage TX signal is cancelled. This TX leakage cancellation may be especially challenging when the wanted signals are broadband. For broadband signals, including multi-carrier, carrier aggregation, multi-band, and so on, the required analog signal processing within feeding forward the TX signal to RX can be much more complex.
More generally, within many telecommunication systems there is the need for signal delay and/or frequency response control over broad band. While DSP can handle very complex signal processing tasks, other times analog signal processing may be useful. For example, within full-duplex radio the TX to RX isolation requirement may be so high that it cannot be achieved without ASP based signal cancellation. Signal processing via continuous-time analog circuitry is much more difficult in practice compared to DSP. There are number of continuous-time domain techniques on attenuating/amplifying, delaying or phase shifting the signal, and they are widely used. However, there are not many techniques on how to combine all required signal processing tasks while maintaining broadband performance.
FIG. 1 illustrates the operation principle of analog adjustable attenuator/gain, phase, and delay. The graphs illustrate what happens to the signal at the output of individual signal processing element over broadband. Ideally the adjustable attenuator/gain is broadband device but it does not alter the delay or phase offset while attenuation/gain is changed. The adjustable phase shifter is also ideally a broadband device but it only adjusts the signal phase offset. Hence, delay and attenuation are not changed while the phase offset is changed. Likewise, ideally the delay adjustment circuit can adjust the delay, namely adjust the phase response slope, but the attenuation and phase offset remains unchanged. All these signal processing tasks can be performed today with several different analog circuit methods. There are significant number of different adjustable attenuator/amplifier circuits, phase sifters and delay adjustment circuits.
There are also hybrid solutions that combine at least two functions. Reflection type I&Q vector modulator, for example, can adjust phase offset and attenuation independently. Hence, in practice, complex gain element can be achieved.
The above basic signal processing functions, or hybrids of those, is that they offer performance that is already limited, even in theory. Adjustable attenuator/gain cannot adjust amplitude slope or ripple over a target band. A phase shifter cannot adjust phase slope or ripple over a target band and a delay element cannot adjust phase offset over a target band, only phase slope is adjusted. These limitations may be unsuitable for broadband high performance.